Such a machine is known for U.S. Pat. No. 3,261,998. This machine is suitable only for low rotational speeds and powers. It is also suitable only as a generator, not as a motor, since in it, sectors of magnetic material provided in the rotor disc extend each to a substantial amount, that is in a large angular region, around the rotor disc.
Disc type generators for high rotational speeds are also known, wherein the rotor, outside the region of the active air gaps, is formed approximately as a disc of uniform strength (Book "Elektrische Maschinen", Authors: Bodefeld and Sequenz, 6th Edition, Page 405; German Pat. No. 228,365). On the other hand, on the region of the active air gaps, the rotor has the form of a flat disc. In order to achieve a great mechanical strength in the rotor construction, the radial height of the active air gap region in these generators is made small compared to the radius of the rotor. Only for this reason, is a low power-weight ratio obtained. This is deteriorated still further by the fact that the energizing windings re arranged radially outside of the rotor and are surrounded on their outside by the edges of the stators, which are connected in U-form so that a very large diameter of the machines results. The large diameter construction results in a large weight for the energizing winding and further results in a great energizing time constant. The long iron path of the magnetic flux also contributes to this.
An electric machine of disc construction style is also known in which the rotor consists of a material having a good magnetic conductivity characteristic and, in the region of the active air gaps, has on both sides thereof teeth formed as ribs with grooves being provided between the ribs (German Pat. No. 1,296,699). In this case, the axial thickness of the rotor diminishes towards the periphery, so that in the region of the active air gaps the rotor is formed approximately as disc of uniform strength. This measure is based upon the problem of achieving a ratio of tooth cross-section to groove cross-section which remains constant over the entire radial height of the active air gap. In this known machine, in departure from that initially mentioned, on each of the two sides of the rotor there are provided two active gap regions lying radially one within the other. The magnetic flux successively penetrates the two radially outer active air gap regions and the two radially inner air gap regions and flows in the radial direction mainly through the yokes of the stators which are remote from the active air gap regions. The energizing windings are arranged in the two stators between the radially outer and the radially inner air gap region. However, this known machine has disadvantages for the two reasons stated hereinafter.
A first reason is that it is necessary not to fall short of a specific minimum diameter of the radially inner active air gap regions. The iron cross-sections which lie in the stator between the alternating-current grooves receiving the alternating-current windings taper radially inwardly toward the shaft by reason of the width of the alternating-current grooves, which remain constant over the radial height of the active air gap regions; and would already terminate on a circle of finite diameter. However, it is not permissable for these iron cross-sections to terminate in a point, since in the region of such points the ratio of the width of the iron cross-section to the width of the alternating-current grooves would be extremely low and unfavorable for the voltage curve form and the efficiency of the machine. Thus, the minimum diameter of the radially inner active air gap region must be selected substantially larger than that diameter on which the possible points of the iron cross-sections would lie. Further the minimum diameter must also be selected sufficiently large so that the winding heads of the alternating-current winding of the radially inner active air gap region can be accommodated within it.
A second reason is that the radial height of the radially inner active air gap region in the known machine is substantially larger than the radial height of the radially outer active air gap region. Both air gap regions are penetrated by the same magnet flux and must therefore have at least approximately the same iron cross-sections lying between the alternating-current grooves. Since the active air gaps have an annular form, their areas and thus also the iron cross-sections, assuming that the edges of the alternating-current grooves extend exactly radially, are in a ratio to one another which is the converse of the ratio of the squares of the mean radii of the active air gaps, that is to say with equal iron cross-section of the two active air gap regions and small mean radius of the radially inner active air gap region, the radial height of the latter must be substantially greater than the radial height of the radially outer active air gap region. Since in fact the edges of an alternating-current groove generally do not extend radially, but parallel with one another, the ratio of the radial heights of the two active air gap regions becomes substantially still poorer. This incidentially becomes more strongly noticeable, the smaller is the minimum diameter selected of the radially inner active air gap region, so that in addition, and for this reason also, the minimum diameter cannot be made very small.
Both reasons set forth above lead to the two active air gaps, lying radially one within the other, necessitating such a great diameter of the machine and thus also of the rotor that the machine in the case of greater powers cannot be constructed for very high rotational speeds. In more recent time there is a need for generators which must, for example, be coupled directly to gas turbines. Rotational speeds can here occur up to 20,000 rpm with powers up to 2,000 kW. At present, no generator is known which masters this rotational speed range with the stated power. Therefore, and at present, this rotational speed is stepped down, with the aid of expensive gearings, in order to drive a generator having a mass of several thousand kilograms.